This invention relates to an optical head and an information medium that are used in optical information processing apparatuses such as optical disks apparatuses, optical card apparatuses an optical tape apparatuses.
In the optical head used in the optical disk apparatuses, etc., an outgoing beam from a semiconductor laser, or the like, is irradiated to an information medium such as an optical disk, by a focussing lens, the reflected beams from the disk are converged again by the focussing lens, are then separated from the outgoing beam from the semiconductor laser by a beam separation optical system, the reflected beams from the disk thus separated are received by optical detectors, a focus error detection signal and a track error detection signal are generated from the optical detector outputs and are supplied to a two-directional lens actuator, for example, and the focussing lens is moved in the direction of the optical axis of the lens and in the radial direction of the disk in order to make focussing control as well as tracking control and at the same time to generate an information reproduction signal from the optical detector outputs.
The structure of the ordinary optical head necessary for accomplishing the function described above such as a light source, the beam separation optical system, the optical detectors, the two-directional lens actuator, and so forth, is firmly fixed to a body. However, to accomplish the focussing control and the tracking control, the focussing lens is movably supported through a spring of the two-directional lens actuator fixed to the optical head body. For this reason, the optical head is not sealed as a whole. A semiconductor laser chip which is relatively compact in size and has a high output is used as the light source of the optical head. The semiconductor laser chip is fixed to a substrate of a package to which lead wires for electrodes are fitted, and is sealed by a cap equipped with a glass window. In this manner, degradation of a light emission end surface of the semiconductor laser chip is prevented and the laser output is stabilized.
One of the focus error detection systems employed for the optical head such as the optical disks utilizes the changes of the shape and beam intensity distribution of the disk reflected beams, receives the beams by a multiple division optical detector and uses the unbalance of its D.C. output signals as the focus error signal.
The astigmatic method disclosed in JP-A-59-58537, for example, provides astigmatism to disk reflected beams by an astigmatic device such as a cylindrical lens. In other words, the astigmatic device provides astigmatism such that the disk reflected beams form two focal lines in mutually orthogonal directions. The disk reflected beams become round at a substantial center of these two focal lines. When the disk reflected beams are received by a quadrature detector, the shape of the reflected beam on the surface of the quadrature detector is substantially round if the disk exists on the focal point and becomes linear in mutually crossing directions if the disk deviates from the focal point. Accordingly, the focus error detection signal can be obtained by adding D.C. output signals of optical detection elements existing at mutually diagonal positions in the quadrature detector and calculating the difference of the two D.C. addition signals.
Another method has been proposed which makes focus error detection by the use of a diffraction grating which forms optical spots at different positions in the direction of the focal depth.
The method of JP-A-1-303632, for example, provides aberration of the longitudinal movement of positive and negative image points to a +1 order beam and a -1 order beam (aberration of the focus error) by the use of a diffraction grating comprising a part of the aggregate of a plurality of concentric grooves the groove pitch of which increases or decreases sequentially. The center of this diffraction grating is not coincident with the major optical axes of the main beam. In other words, since the concentric grooves of the diffraction grating are eccentric with respect to the major optical axis of the main beam, the +1 order beam and the -1 order beam are emitted in directions opposite to the major optical axis of the main beam. These +1 order beam and -1 order beam are focused by a focussing lens as two side spots at different positions in the direction of the focal depth relative to the main beam. The light power of the reflected beams from these two side spots is modulated by a signal recorded on the optical disk and the degree of modulation is detected by the optical detection device and an envelop detection circuit. Since the degree of modulation by the two side spots changes in accordance with the focus error of the optical disk, the focus error signal can be obtained by calculating the difference between the two degrees of modulation.
Next, an example of a beam separation optical system for separating a reflected beam from a recording medium and a beam from a light source such as an optical disk will be explained.
According to JP-A-59-58537 described above, for example, the beam separation optical system comprises a polarizing prism and a quarter wavelength plate. In this beam separation optical system, the polarizing direction of the linearly polarized beam turns by 90.degree. when the laser beam reciprocates on the quarter wavelength plate. Therefore, the two beams can be separated completely by the polarizing prism. The polarizing prism that is used generally is a cubic polarizing beam splitter produced by sandwiching dielectric multi-layers between two triangular prisms, and separates the reflected beam in the orthogonal direction.
The optical head of the compact disk, etc, does not use the quarter wavelength plate so as to reduce the number of optical components but uses a half prism or a half mirror for separating about the half of the disk reflected beam in the orthogonal direction as the beam separation optical system.
Besides the prior art described above, JP-A-2-216629 discloses a compact optical head having a structure wherein a diffraction grating or a holographic device is inserted between a semiconductor laser and a focussing lens, and a reflected beam from an optical disk is separated from an outgoing beam from the semiconductor laser at a very fine angle, so that an optical detector can be mounted to the semiconductor laser adjacent thereto.